Uranium is a metallic element of the actinide series currently used inter alia as fuel in nuclear reactors for electric power generation. Uranium is generally recovered from ores containing uraninite (UO.sub.2), coffinite (USiO.sub.4 OH) or carnotite. The hexavalent uranium and uranyl ion (UO.sub.2.sup.++) are soluble in aqueous acidic solutions. Conventional recovery techniques from ore include leaching with sulphuric acid under oxidizing condition to obtain a uranyl sulphate solution, which is subsequently purified by solvent extraction and converted to uranium dioxide either in the natural or enriched form.
A technique well known in the uranium ore processing industry is the strong acid strip process. This process is used for the removal of uranium from organic solution during solvent extraction. Details of the process are displayed by Swider in "Stripping of Tertiary Amine for Uranium Recovery", presented at the International Symposium on Uranium and Electricity, Saskatoon, Canada, Sep. 18-21, 1988. The chemistry of the strong acid strip process is based on the chemical equilibrium of the species involved. The reactions for extraction of uranium involve an anionic extractant such as a tertiary amine, which is a component of a mixture of generally called "organic solution" or "organic phase", and comprises a tertiary amine such as a saturated straight chain trialkylamine, example of which being manufactured and sold by Henckel Corp. under the trademark Alamine 336, a modifier such as isodecanol, and a carrier such as kerosene. The chemistry of the extraction and subsequent stripping can be illustrated as follows:
Extraction: EQU 4(R.sub.3 NH)HSO.sub.4 +UO.sub.2 (SO.sub.4).sub.3.sup.4- .revreaction.(R.sub.3 NH).sub.4 UO.sub.2 (SO.sub.4).sub.3 +2H.sub.2 SO.sub.4 + EQU 2SO.sub.4.sup.2- 2(R.sub.3 NH)HSO.sub.4 .revreaction.(R.sub.3 NH).sub.2 SO.sub.4 +H.sub.2 SO.sub.4
Stripping: EQU (R.sub.3 NH).sub.4 UO.sub.2 (SO.sub.4).sub.3 +2H.sub.2 SO.sub.4 .revreaction.4(R.sub.3 NH)HSO.sub.4 + EQU (UO.sub.2.sup.2 +SO.sub.4.sup.2-) (R.sub.3 NH.sub.2)SO.sub.4 +H.sub.2 SO.sub.4 .revreaction.2(R.sub.3 NH)HSO.sub.4
By increasing the acid concentration in the strip solution, uranium is transferred into the strip solution. A five stage typical counter-current mixersettler stripping plant is currently in commercial operation.
The performance of the current strong acid strip process is however subject to constraints. Acid transfer during stripping is a nominal 1 g H.sub.2 SO.sub.4 per gram of U.sub.3 O.sub.8 stripped. Further, a free acid strength must be maintained in the pregnant strip to preserve efficient stripping, an appropriate number of stages of contact is required to achieve process performance in terms of pregnant strip and stripped organic uranium concentration, and the degradation or de-activation of the organic solution must be controlled by avoiding excessive acid strength in the strip solution or excessive temperature during stripping contact.
Even though the above process has proved to give satisfactory results, improvement of the operating parameters of the process would certainly be desirable. For example, it would be most advantageous to improve the sulphuric acid to uranium oxide ratio, which is presently rather high.